Euthinization and overpopulation of companion animals

Have you ever wondered what happens to the stray companion animals that we see roaming the streets? If they are not taken in by caring citizens they are either put in shelters, adopted but usually they are killed.

What is a companion animal?
How many cats and dogs enter shelters a year?
How many cats and dogs are euthanized by shelters each year?
How many cats and dogs are adopted from shelters each year?
What percent of shelter animals are purebred?
How can we reduce the number of neglected companion animals?
How many shelters are there in the United States?

I have a soft spot in my heart for animals. I always have since I was a child. Once, I was walking and found a dying baby bird on the sidewalk. It was barely breathing and looked like it had been pecked by other birds. I read somewhere that a mother bird will not feed her baby if she doesn't recognize the scent. I went upstairs to my apartment, got a towel and a shoe box and to put the bird in. When I got inside I was sure the bird had not had any water. My mom is a nurse so I got a syringe filled with sodium chloride, which is just salt water, and put it in the baby birds mouth. I was so excited to see him drinking the water. I waited for my mom to take the bird to the nearest vet. When we got there I was very sad and disappointed when they told me they wouldn't take care of the bird because it was going to die either way. It made me wonder what other animals are turned down and how they are treated when no one wants to adopt or take care of them. I am broadening my knowledge of companion animals and euthinization because I want to know why unwanted or animals that aren't adopted are being killed in order to help the reader understand that there are simple solutions to this problem.